Yom Kippur is the holiest day of the Jewish year because it is the day that the Torah sets aside for us to reckon with ourselves. We are commanded to give an accounting--before our fellow men and before God--for our actions in the previous year. We must make amends to both for our misdeeds. And since none of us is perfect, every one of us has things to atone for.
Yom Kippur's power stems from a basic assumption that forms its core. That assumption is that we are all moral agents. We all have to make an accounting.
This basic assumption is the most liberating notion ever created. Moral agency is what makes us free. It doesn't matter how wretched or rich our external circumstances, the fact that the Torah enjoins all of us to take responsibility for our behavior means that as far as God is concerned, we are not slaves and never will be slaves.
The converse is also true.